Does it mean that gzip is a legacy compression than should be replaced with zstd where possible?
gzip will stay with us because of ubiquity, just as we mostly use image formats from the dot-com era. But if you control both sides, zstd is a big upgrade.
gzip compression levels are almost useless and typically result in very little actual compression ratio differences, but typically with massive CPU usage differences.
zstd doesn't do everything better than gzip though; I believe gzip still offers somewhat faster compression for equal compression ratios last time I looked (but decompression is much faster). I mostly replaced gzip with zstd myself, but there are still scenarios where gzip might be preferable.
Compression time CPU is lower for equivalent ratios. Maximum compression is better. Decompression CPU is much lower (i.e. faster), and that is independent of the level used at compression time.
Overall, zstd still came out as the clear winner (also compared some other compression tools), so I went with that.